I am trying to make a point to someone that just because two plants share a family and one plant is safe for human consumption, it does not follow that the other plant also is safe for human consumption. Can anyone provide an example I can use as proof?

Well, given that often enough a plant is considered "edible", while parts of THE SAME PLANT are already poisonous (see the potato), it stands to reason that the definition of "safe for human consumption" is far too lax of a term to even allow for a clear "no there aren't any". [Also there's a ton of counterexamples, like Byans below]
– HobbamokSep 25 at 10:30

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"family" in itself is such a borad term, that even a pure explanation to what a plant family is accidentallly provided an answer to your question: maximumyield.com/definition/785/plant-family [it's the nightshades again though, but this shows how valid your point is]
– HobbamokSep 25 at 10:32

Other members of the family are tomatoes, peppers, potatoes, and more.

Plant families can be massively diverse, and toxicity doesn't really have much relationship to family. Most of the compounds that are found in plants that are toxic are found in other non-toxic plants as well: dose is crucial.

Potato is also an example of a plant that is considered non-toxic, but parts of the plant are toxic and - even worse - the tubers can become toxic. The green parts (leaves, stems, etc) contain a poisonous alkaloid. If a potato in storage starts to sprout, the sprouts and eyes are often toxic. If the potato itself enlarges while sprouting, it will also typically have elevated toxicity.
– PeterSep 23 at 11:14

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@Peter: Another common example is rhubarb. The stems are edible (at least when cooked), but the leaves are toxic due to concentrations of oxalic acid.
– jamesqfSep 23 at 18:23

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I believe that if the potato tuber is exposed to sunlight as the plant grows, it will also become green and poisonous, like the leaves.
– ArthurSep 24 at 14:40

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Indeed, not just potatoes - tomatoes are toxic as well (leaves, unripe fruit...), and arguably, so are peppers (not "deadly" toxic, but the capsaicin is definitely another thing "designed" to make mammals avoid eating the fruit; it's not spicy to birds, who are the primary dispersers of pepper seeds) and eggplant (though that one has been pretty effectively domesticated to be safe as long as it's not raw). But it still definitely counts as "safe for human consumption" as long as you avoid the poisonous parts.
– LuaanSep 25 at 7:55

For what it's worth, part of the cashew plant is actually lethally toxic as well.
– Austin HemmelgarnSep 23 at 2:16

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@AustinHemmelgarn which parts are toxic? Any sources?
– MaheshSep 23 at 11:33

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The shell of the nut itself,, which is why they're almost always sold with the shells already removed.. Just like poison ivy, it has oils that cause contact dermatitis, and if ingested can cause potentially lethal internal reactions in the digestive tract. Also like poison ivy,, the smoke from burning the shells is really dangerous because the oils don't completely combust.
– Austin HemmelgarnSep 23 at 12:03

The poison ivy, oak, and sumac all used to be in the genus Rhus alon gwith edible sumac used as a spice.
– kingledionSep 25 at 16:14

Fungi are not plants and you've tagged this as botany, so this is perhaps off-topic, but I feel like it might help you make your point: the genus Amanita contains extremely toxic species (A. phalloides), highly regarded edible ones (A. caesarea) as well as psychoactive ones (A. muscaria).

Many common names are reused for different plants. This is true for "buttercup" which is also used for daffodils and pink evening primrose. Thanks for including the binomial.
– Dennis WilliamsonSep 25 at 19:53